The present disclosure relates to a tomographic image generation device, a tomographic image generation method and a tomographic image generation program for obtaining a plurality of projection images of a subject by imaging the subject with different radiation source positions, and generating a tomographic image from the projection images.
In recent years, in order to more closely observe an affected part of the body with a radiographic imaging apparatus using radiation, such as x-ray or γ-ray, tomosynthesis imaging has been proposed, in which imaging is performed by applying radiation to the subject from different radiation source positions by moving the radiation source, and the thus obtained projection images are added up to generate a tomographic image in which a desired slice plane is emphasized. In the tomosynthesis imaging, a plurality of projection images are obtained by imaging a subject with different radiation source positions by moving the radiation source in parallel with the radiation detector or along a circular or ellipsoidal arc trajectory depending on characteristics of the imaging apparatus and necessary tomographic images, and the projection images are reconstructed to generate a tomographic image using a simple reverse projection method, or a reverse projection method such as a filter reverse projection method.
However, with the tomosynthesis imaging, angles at which the radiation can be applied to the subject are limited, and there may be cases where a tomographic image reconstructed from the projection images by a reverse projection method, for example, includes an artifact which is a virtual image of a structure in an area of the tomographic image where the structure in the subject is not actually present. More specifically, the reverse projection may introduce an artifact of a structure in an area where the structure is not actually present of the tomographic image of a slice plane, which is different from a tomographic image of a slice plane where the structure is present. When such an artifact is too visible, it is difficult to see a structure, such as a lesion, which is necessary for diagnosis.
During the tomosynthesis imaging, the radiation is applied to the subject at a plurality of times, and a radiation dose as low as possible is used for each time to reduce the radiation exposure of the subject. However, a low radiation dose results in more quantum noise of radiation in the projection images obtained by the imaging, which in turn results in more visible noise in the reconstructed tomographic image.
Various techniques for reducing such an artifact or noise have been proposed. For example, Japanese Unexamined Patent Publication No. 2013-000261 (hereinafter, Patent Document 1) has proposed a technique which involves: calculating a similarity between each pixel of a reference projection image which is one of projection images and the corresponding pixel of each of the other projection images, which pixels are cumulatively added on the same position on a tomographic image; calculating a weighting factor for each pixel of the projection images such that the weighting factor is greater when the similarity is higher; and reconstructing the tomographic image by calculating a cumulative addition of products calculated by multiplying each pixel value of each projection image with the corresponding weighting factor.
Besides the above-described technique, techniques called an algebraic reconstruction method or a iterative approximation reconstruction method have also been proposed. These techniques calculate a tomographic image such that images formed by projecting the reconstructed tomographic image agree with the actually taken projection images. These techniques allow incorporating various mathematical models in the reconstruction, thereby allowing taking the artifact correction, the noise reduction, etc., into account to generate a tomographic image with suppressed artifacts and reduced noise.
However, the technique taught in Patent Document 1 and the reconstruction process such as the iterative approximation reconstruction method have a drawback that they require very long calculation time. To address this problem, a technique for reducing artifacts while reducing the calculation time has been proposed, which involves: generating a plurality of bandlimited images with different frequency response characteristics from projection images; performing nonlinear transformation on the bandlimited images such that portions that exceed a predetermined value of the bandlimited images become small; adding up the bandlimited images after the nonlinear transformation to generate a plurality of transformed images; and reconstructing a tomographic image from the transformed images (see Japanese Unexamined Patent Publication No. 2013-031641, hereinafter, Patent Document 2).
Japanese Unexamined Patent Publication Nos. 2005-013736 and 11(1999)-339050 (hereinafter, Patent Documents 3 and 4, respectively) have proposed techniques to improve the calculation speed for reconstructing a tomographic image by setting the pixel size of the tomographic image the same as the pixel size of the radiation detector regardless of the height of the slice plane.